1. Field of the Invention
This invention relates to a thermo-sensitive flow rate sensor employing a flow rate detecting device which has a heating element and which is used for measuring flow velocity or flow rate of a fluid on the basis of a heat transfer phenomenon where a heat is transferred from the heating element or from a part heated by the heating element to the fluid. This invention is applied to, for example, a thermo-sensitive flow rate sensor for use in measuring an intake air amount in an internal combustion engine.
2. Description of the Related Art
FIGS. 13 and 14 are a side sectional view and a plan view of a conventional flow rate detecting device described in, for instance, Japanese Unexamined Patent Publication No. 4-230808 Official Gazette, respectively.
In the device shown in FIGS. 13 and 14, a plate-like substrate 1 is constituted by a silicon semiconductor. A thin-layer-like diaphragm portion 13 is integrally formed in a central portion of the top surface of the plate-like substrate 1. This diaphragm portion 13 is formed as follows. For example, anisotropic etching is performed on the plate-like substrate 1 from the back-surface side thereof. Thus, a part of the plate-like substrate 1 is removed so that a cavity 33, which does not reach the top surface of the plate-like substrate 1 and which has a trapezoidal section, is formed in a central portion of the back surface of the plate-like substrate 1.
Further, a thin film heating element 4 is formed on the central portion of the top surface of the diaphragm portion 13, which is constituted by forming a part of the plate-like substrate 1 like a thin layer. Moreover, thin-film temperature measuring elements 34 and 35 are respectively placed along both sides of the heating element 4 in such a manner as to be at a predetermined distance therefrom and to be symmetrical with respect thereto. Furthermore, linear holes 36a and 36b penetrating the diaphragm portion 13 are formed between the heating element 4 and the temperature measuring elements 34 and 35 in such a way as to extend along the longitudinal direction of the heating element 4. Further, a plurality of holes 37a and 37b penetrating the diaphragm portion 13 are formed outside the temperature measuring elements 34 and 35 in such a manner as to extend along the longitudinal directions thereof. Similarly, holes 38c and 38d penetrating the diaphragm portion 13 are bored therein at both sides in the longitudinal direction of the heating element 4. Moreover, holes 40c and 40d penetrating the diaphragm portion 13 are bored at both sides in the longitudinal directions of the temperature measuring heating elements 34 and 35. These holes are formed by techniques, such as photolithography, wet etching and dry etching, in such a way as to have rectangular sections.
When an amount of electric current fed to the heating element 4 is controlled by the conventional flow rate detecting device in a manner such that the temperature of the heating element 4 will rise to a predetermined value which is higher than the temperature of the fluid by a predetermined number of degrees, the temperature measuring elements 34 and 35 have the same temperature if the fluid does not move (namely, the flow velocity thereof=0).
When the fluid is caused to move in the direction of an arrow 6, the temperature of the temperature measuring element 34 placed upstream of the temperature measuring element 35 falls below the temperature of the element 34 in the case that the flow velocity =0. As the flow velocity increases, the temperature of the element 34 falls. On the other hand, the temperature of the temperature measuring element 35 does not fall to that of the upstream temperature measuring element 34 at the same flow rate. The flow rate of the fluid, therefore, can be measured by obtaining a quantity corresponding to the difference in temperature between the temperature measuring elements 34 and 35 by means of the device having a Wheatstone bridge circuit into which the temperature measuring elements 34 and 35 are incorporated.
The Japanese Unexamined Patent Publication No. 4-230808 Official Gazette describes that the conventional flow rate detecting device obtains the advantages that variation in output thereof due to deposition of dust thereto is reduced by boring holes therein to thereby lower the flow rates of heat flows flowing from the heating element 4 to the temperature measuring devices 34 and 35 and to thus lower the temperatures of the elements 34 and 35, and that the sensitivity thereof is enhanced because the flow rate of flow of heat conducted from the heating element 4 to the plate-like substrate 1 can be decreased.
Meanwhile, when the heating element 4 is energized and caused to generate heat, when the flow velocity of the fluid increases, when a pressure is exerted upon the flow rate detecting device, or when the flow rate detecting device is subjected to large vibrations, stress is produced in the diaphragm portion 13.
In the conventional flow rate detecting device, all of the holes 36a, 36b, 37a, 37b, 38c, 38d, 40c and 40d are formed in such a way as to have rectangular sections. Thus, the conventional flow rate detecting device has the problem that the aforementioned stress is concentrated on the corner portions of the holes, so that the diaphragm portion 13 is easy to break at these corner portions. Especially, in the case that a plurality of holes are provided in the diaphragm portion 13 so as to ensure heat insulation, the number of fragile parts increases.
Further, in the conventional flow rate detecting device, the holes are bored in the vicinity and upstream of the heating element 4 and the temperature measuring elements 34 and 35. Thus, when this device is used over a long period, dust contained in a fluid to be measured accumulates on the downstream inner surface of the holes. Consequently, the conventional flow rate detecting device has the additional problem that the condition of the flow of the fluid flowing on the surface of the diaphragm 13 changes and the detecting characteristics of the device vary.
For instance, when this conventional flow rate detecting device is employed in an intake air flow rate sensor for use in controlling a fuel for an automotive engine, the following trouble occurs.
The automotive engine causes vibrations, whose acceleration ranges from 40 Gal to 50 Gal. Further, the flow velocity of intake air sometimes reaches 200 m/s or more. Moreover, when the engine backfires, a pressure being close to 2 atm may be applied to the device. In the case that the conventional flow rate detecting device is subjected to such mechanical stress, this device easily breaks from the hole portions formed therein.
On the other hand, the intake air of the internal combustion engine flows through an air cleaner element disposed upstream of the intake air flow rate sensor. Dust particles of a few microns in size pass through the air cleaner element and then deposit to the downstream inner surface of each of the holes. This changes the flow of air flowing downstream from the holes. Consequently, the flow rate detecting performance of the device is deteriorated.
This invention is accomplished to solve the aforementioned problems of the conventional device. Accordingly, an object of the present invention is to provide a thermo-sensitive flow rate sensor with high sensitivity and reliability.
To achieve the foregoing object, according to an aspect of the present invention, there is provided a thermo-sensitive flow rate sensor having a flow rate detecting device that comprises a plate-like substrate, a part of which is removed so that a space is provided therein, a diaphragm portion formed like a thin layer above the aforesaid space in such a manner as to be integral with the aforesaid plate-like substrate, and a heating element constituted by a thermo-sensitive electrically resistant film formed on the aforesaid diaphragm portion, the aforesaid thermo-sensitive flow rate sensor being adapted to measure the flow rate of a fluid, which is to be measured, according to an amount of heat transferred to the aforesaid fluid from a part heated by energizing the aforesaid heating element. wherein a plurality of holes are provided in an outer peripheral portion of the heating element so as to penetrate the diaphragm portion, the holes being shaped in such a way as to have obtuse corner portions or to have substantially no corner portions.
According to another aspect of the present invention, there is provided a thermo-sensitive flow rate sensor having a flow rate detecting device that comprises a plate-like substrate, a part of which is removed so that a space is provided therein, a diaphragm portion formed like a thin layer above the aforesaid space in such a manner as to be integral with the aforesaid plate-like substrate, and a heating element constituted by a thermo-sensitive electrically resistant film formed on the aforesaid diaphragm portion, the aforesaid thermo-sensitive flow rate sensor being adapted to measure the flow rate of a fluid, which is to be measured, according to an amount of heat transferred to the aforesaid fluid from a part heated by energizing the aforesaid heating element, wherein a plurality of holes are provided in an outer peripheral portion of the heating element which is other than a part located upstream of the heating element so as to penetrate the diaphragm portion.
According to still another aspect of the present invention, there is provided a thermo-sensitive flow rate sensor having a flow rate detecting device that comprises a plate-like substrate, a part of which is removed so that a space is provided therein, a diaphragm portion formed like a thin layer above the aforesaid space in such a manner as to be integral with the aforesaid plate-like substrate, and a heating element constituted by a thermo-sensitive electrically resistant film formed on the aforesaid diaphragm portion, the aforesaid thermo-sensitive flow rate sensor being adapted to measure the flow rate of a fluid, which is to be measured, according to an amount of heat transferred to the aforesaid fluid from a part heated by energizing the aforesaid heating element, wherein a plurality of holes are provided upstream and downstream of the heating element and along the heating element so as to penetrate the diaphragm portion, the holes provided upstream of the heating element being spaced further apart from the heating element than the holes provided downstream of the heating element.